The importance of making various types of documents safe from nefarious duplication is readily apparent. The development of the color copier has resulted in significant concern in this regard. The quality of color reproductions that can be made with a color copier has become so good that it may be very difficult to distinguish the original documents from the color reproductions. Even if a reproduction is not an exact copy, the reproduction often appears reasonably authentic in the absence of the original for comparison purposes. As a consequence, there has been concern that color copiers could be used to reproduce security documents, such as checks, stock certificates, automobile title instruments, and other documents of value, for illegal purposes. This concern has been heightened with the advent of desk top publishing software and hardware, including personal computers and scanners. Such desk top publishing systems allow sophisticated image processing and printing not previously generally available.
Many techniques have been developed to prevent improper reproduction of security documents. One of the most successful is the use of a hidden warning message which is readily apparent on reproduced copies of a document, but which is invisible, or nearly so, on the original document.
Examples of this technique are shown in U.S. Pat. Nos. 4,227,720 and 4,310,180. A single tone warning phrase and a single tone background pattern are used. Tone refers to the visual effect produced by solid ink coverage or by halftone dots, bars, or marks which cover a portion of a printed area and which usually have a frequency that is measured in dots, lines, or marks per inch. Halftone dots, bars, or marks may be more or less uniformly distributed over an area to produce the visual effect, i.e., less than full tone, of a lighter overall color with the use of a darker color ink printed at less than full area coverage. The warning phrase and background pattern area tones are of different frequency and are made up of dots, bars, or marks of differing size but they are selected to provide similar appearance to the eye of a casual observer. A less than full tone effect may also be produced by full area coverage of a paler color of ink than the darker color of ink used for the halftone dots, bars, or marks.
Because the tone of the warning phrase and the tone of the background pattern are selected to be the same, these two areas have much the same visual impact on an observer of the original document, and the warning phrase is not readily perceived. The optics of color copiers have typically been unable to reproduce relatively small halftone dots, lines or other elements. As a consequence, reproduced copies of the original document will have a noticeable warning phrase.
A camouflage pattern is sometimes utilized to obscure the warning phrase further. The camouflage pattern may be defined by areas in which the dots, bars, or marks have been deleted from both the warning phrase and the background pattern. The camouflage pattern may also be defined by a pattern of dots, bars, or marks which are smaller than or larger than those used in the background pattern and the warning phrase, or by areas of complete coverage of a paler ink.
In recent years, color copiers have been improved substantially. These new color copiers have made the above technique less effective in protecting documents. By manipulating the control settings on such copiers, copies can be made of such documents in which the warning phrase does not appear on reproductions when some of the most commonly used frequency and size combinations are used. Furthermore, desk top publishing systems now available in conjunction with laser printers, offer additional possibilities for unauthorized copying. Therefore, it is clear that improvements in this technique are desirable.
One such improvement is shown in U.S. Pat. No. 4,351,547, which also utilizes a single tone background and a single tone cancellation phrase. In this improved technique, the warning phrase is not defined by dots or elements of the same size and frequency. Instead, the warning phrase is defined by an alternating dot pattern which includes large dots of lower frequency than the background tone, and small dots located in exact registry with the large dots.
U.S. Pat. No. 4,579,370 represents another improvement in the use of a hidden warning phrase. The background and warning phrase are each made up of halftone dots of two pairs of dot sizes. For example, the background might be made with about 50% of 130 lines per inch 0.005 inch diameter, and the balance of 130 lines per inch, 0.006 inch diameter; the cancellation term might be made with about 50% of the dots of 65 lines per inch, 0.010 inch diameter and the balance of 65 lines per inch, 0.012 inch diameter. This provides additional protection for documents against improper copying.
These methods have generally been successful in protecting documents at most copier settings. However, by adjusting the settings for sharpness and lightness/darkness it has still been possible on some copiers for a skilled individual to produce a copy in which the warning phrase is not visible.
Therefore, there remains a need in the art for a security document which provides improved protection against copying over a wide range of copier settings, or against manipulation using desk top publishing systems.